A significant body of scientific evidence has shown that long-term ocular exposure to solar radiation is accompanied by specific detrimental physiological effects. Scientific studies have primarily considered two types of physiological effects: ocular effects (related to the eye) and erythemal effects (related to the skin). The statistical significance of these effects, including cataracts and other ocular damage mechanisms, correlates to increasing age and cumulative exposure to ambient sunlight. In a similar manner, the incidence of skin cancer and related cutaneous pathologies is also correlated to cumulative exposure to solar radiation. The aging population demographic in the industrialized nations and the decline in upper atmospheric ozone concentration have heightened the significance of this issue.
Leading scientific and regulatory organizations in several countries have conducted extensive investigations and have developed quantitative ocular radiation exposure guidelines. The intent of these standards is to provide a scientific basis for evaluating ocular exposure parameters and to assist in the control of health hazards. The technical basis of these standards is the fact that certain wavelengths within the solar radiation spectrum are more damaging than others. Ultraviolet (UV) and deep-blue wavelengths are prime examples of this effect. At a given intensity and time of exposure, the ultraviolet and blue wavelengths are more damaging to the human eye than other wavelengths in the visible band. For this reason, minimizing ocular exposure to UV radiation has long been an accepted principle in the occupational health sciences.
Generally, however, technical standards regarding the spectral transmission properties of commercial products such as eyewear and glazing materials have not been widely imposed by regulatory agencies or adhered to by manufacturers. A relevant example of this is the inaccurate claims of “100% UV protection” which persist throughout the retail stream of commerce for sunglasses. Currently, there is no widely accepted metric by which the safety attributes of such products can be accurately described.
Ambient solar radiation at the earth's surface is comprised of a broad electromagnetic spectrum, including ultraviolet, visible, and infrared wavelengths. Only the visible component of the sunlight can be perceived by the human eye. Classically, visible light is defined as electromagnetic radiation with wavelengths between approximately 400-700 nanometers (nm). The different visible wavelengths correspond to the various colors which are perceived by the human eye. The eye perceives purple and blue colors at the shorter-wavelength part of the visible spectrum (400-470 nm), and perceives red at the longer-wavelength part of the visible spectrum (660-700 nm). Green and yellow colors are perceived at the wavelengths in the middle of the visible spectrum. Visible wavelengths represent only a small fraction of the solar spectrum which penetrates the earth's atmosphere and reaches the surface of the planet. Ambient solar radiation at the Earth's surface includes a spectrum of wavelengths from approximately 300 nm (ultraviolet light) to infrared light at wavelengths exceeding 1000 nm.
Solar radiation with wavelengths below 300 nm is substantially absorbed in the upper atmosphere. This is fortunate since shorter-wavelength ultraviolet radiation is very damaging to organic matter. Without this atmospheric shielding effect, shorter-wavelength UV radiation would severely damage or extinguish many terrestrial life forms. Ultraviolet radiation with wavelengths between 185-280 nm is commonly called “UVC”. This band of UV radiation is also called “germicidal UV” since it can quickly destroy pathogenic organisms even at moderate flux densities. Specialized UVC lamps are used in numerous anti-bacterial applications such as sterilization of potable water. Since the earth's atmosphere absorbs the shorter-wavelength UV radiation, exposure to UVC is normally associated with artificial lamp or laser sources which require special protective measures.
The commonly used terms “UVA” and “UVB” relate to ultraviolet radiation with wavelengths of 315-400 nm and 280-315 nm, respectively. Owing to shielding effects of our atmosphere, the UV component of solar radiation measured at the earth's surface occurs in a range of 300 to 400 nm in wavelength. High-quality eyewear should be designed to attenuate the UVA and UVB components of natural sunlight since these represent the elements of the ambient solar radiation spectrum which are most damaging to the human eye.
In the U.S., the American Conference of Governmental Industrial Hygienists (ACGIH) has conducted extensive studies of ocular radiation exposure issues. The research work conducted by the ACGIH has focused on technical criteria for evaluating exposure to a substantial range of substances and radiation types. In this regard, the ACGIH has published data supporting its Biological Exposure Indices (BEIs) and Threshold Limit Values (TLVs) as guidelines to assist in the control of health hazards. Threshold Limit Values (TLVs) relating to values for ocular exposure, include “spectral weighting functions” for specific wavelengths. In Europe, the International Committee on Illumination (CIE) in Paris has completed similar studies which have resulted in algorithms for calculating the “luminous transmittance” and “coloration factors” from spectrophotometric measurements of protective lenses.
This same methodology has been applied to infrared (IR) wavelengths. The ACGIH has developed spectral weighting functions and TLVs for “retinal thermal injury” from visible and IR wavelengths. Unlike UV radiation, the visible and IR wavelengths are more efficiently transmitted through the eye onto the retina. Historically, “glass blower's cataract” is a term used to describe the ocular damage which results from occupational exposure to the visible and IR radiation that is produced by high-temperature glass furnaces.
Although the specific ocular damage mechanism for visible and IR radiation differs from that of the UV wavelengths, the same principle holds true: the severity of the cumulative ocular damage effects varies as a function of wavelength and flux density or luminous intensity. In simple terms, certain wavelengths are more damaging than others at a given intensity. As shown in FIG. 1, the threshold exposure for ocular damage is dependent upon the wavelength of incident radiation.
For instance, the ACGIH “blue-light hazard function” (the spectral weighting function for retinal photochemical injury from chronic exposure to blue light) for light with a wavelength of 440 nm is 1000 times larger than for 590 nm wavelengths. In a similar manner, the ACGIH “relative spectral effectiveness” (Sλ) factor for ultraviolet light with a wavelength of 300 nm is 10,000 times higher than the S value at 400 nm. The standard Erythemal response curves, shown in FIG. 2, depict erythemal response or reddening of the skin to UV radiation, whereby UV radiation at a 300 nm wavelength has 25-100 times more erythemal potency than 315 nm radiation, as shown in FIG. 2.
The CIE concept of luminous transmittance is basically a mathematical model which embodies “photopic efficiency” and “relative energy” factors in order to quantify the total integrated brightness of ambient radiation, as perceived by a “standard observer” wearing lenses with a specific transmission spectrum. CIE luminous transmittance is calculated as the product of spectral transmittance (Tλ), photopic spectral luminous efficiency (Vλ), and the relative energy value of Standard Illuminant C (Sλ). The CIE has determined the values of Vλ and Sλ within the optical spectrum between 380-780 nm; the mathematical product of these two factors [(Vλ)(Sλ)] is provided in the CIE standard at 10 nm intervals. Luminous transmittance is derived by first employing a conventional spectrophotometer to measure the transmission spectrum of the lens or substrate being characterized. The measured transmission values (Tλ) at 10 nm intervals are then multiplied by the respective values of [(Vλ)(Sλ)]. The mathematical products of these operations are then summed over the range of 380-780 nm and divided by 100,000 in order to derive the total (percent) luminous transmittance.
The CIE also promulgates a permutation of this model in the form of “coloration factors”. Examples of these include the “Red Signal Visibility Factor” and the “Violet Factor”. These variations of the CIE model apply the same basic calculation method to discrete portions of the spectrum. Respectively these are the red (620-780 nm) and violet (420-460 nm) parts of the visible spectrum. These “coloration factor” models are intended to address signal/sign visibility issues from the point of view of a “standardized” human observer.
In contrast to the present invention, however, the ACGIH work does not teach measured transmission spectra of protective lenses or other devices which attenuate an ambient radiation spectrum which has been characterized. The ACGIH document does not teach mathematical terms or reference factors of any kind which address the shielding or attenuation effects which are achieved by protective eyewear, fabrics, cosmetics, industrial applications and other transparent, translucent, or opaque optical materials which are designed to provide protection from ambient radiation. In addition, the CIE work does not teach the biological impact of the radiation spectrum. In addition, the CIE document contains no measurements or reference factors which address the ocular or erythemal effects of exposure to radiation spectra.
U.S. Pat. No. 5,949,535 to Hall teaches quantitatively rating protection values against solar radiation for eyewear. Hall further proposes that sunglass protection factors (SPFs), eye protection ratings (EPRs) and eye protection factors (EPFs) can be derived from the light transmission values of the sunglass lenses. Hall teaches that these factors are based on the average value of light transmission across a range of wavelengths, (280-400 nm for UV and 400-500 nm for blue light). These average transmission values are then converted to a linear (0-100) scale which yields a single numerical value for the measured property.
However, as shown in FIGS. 1 and 2 herein, the values of the Threshold Radiant Exposure and Relative Effectiveness are not linear as a function of wavelength. Instead, the light transmission values in the wavelength ranges of UV and blue light transmission, as shown in FIGS. 1 and 2 herein, are exponential or sinusoidal in nature. Hall does not take the non-linear effects of the light transmission values into account, but instead assumes that the exponential increase in source intensity as a function of wavelength is offset by an exponential decrease in toxicity of the longer wavelengths and further teaches that the net ocular damage is constant over the UV and blue wavelength bands (280-400 nm and 400-500 nm, respectively).
Therefore, Hall does not fully consider the significant variations in ambient solar irradiance, lens transmission, and physiological effectivity as functions of wavelength. In addition, Hall does not consider the physiological effects of infra-red wavelengths, the integrated physiological effects of the ambient solar spectrum, including UV, blue, and infra-red wavelengths, and the intensity and spectral distribution of the ambient illumination.
U.S. Pat. No. 5,971,537 to Fukuma, et.al (hereinafter Fukuma) teaches that the refractive properties of eyeglass lenses can be measured by an optical/electronic “lens specifying apparatus” which can also be used to measure spectral transmission properties in the UV and visible wavelengths. Fukuma further teaches that the invention can be used to measure and display the spatial distribution of these properties at different locations on the lens in order to characterize the properties of a “progressive” lens by mapping its refractive and spectral transmission properties at a plurality of points on the lens. However, Fukuma does not teach conducting and analyzing spectrophotometric measurements of eyeglass lenses in order to calculate indices which accurately characterize the physiological effects of transmitted radiation. In addition, Fukuma does not discuss transmission properties in relation to analyzing or rating the physiological impact of spectral transmission values of the lenses or optical substrates.
What is needed is a method of quantitative analysis of the total ocular and erythemal hazard of transparent or translucent materials. What is needed is a method of deriving and applying a quantitative index which defines the amount of ocular or erythemal hazard that is protected by the measured material properties of an intervening medium.